The present invention relates to a marine riser system for use in off-shore drilling operations and to a method for installing the same.
Off-shore drilling operations that are carried out by means of drilling equipment aboard a floating drilling unit, such as a drill ship or a semi-submersible, often make use of a marine riser extending between the wellhead of the well that is being drilled in a subsea formation and the drill ship. Such marine riser is a tubular means made up of a plurality of tubular sections that are connected in end-to-end relationship. The riser, if installed between a ship and a wellhead, allows return of the drilling mud with drill cuttings from the hole that is being drilled. Also, the marine riser is adapted for being used as a guide means for lowering equipment (such as a drill string carrying a drill bit) into the hole. A riser may comprise concentric tubular strings of pipe as shown in U.S. Pat. No. 3,721,292.
The lower end of a marine riser is detachably connected to the wellhead and the upper end thereof is connected to the drill ship by means of a heave compensator which allows the marine riser to be tensioned by the drill ship without allowing overloading of the marine riser and the cables by which the riser is supported from the vessel by heave of the drill ship. If desired, a substantial part of the lifting force required to tension the marine riser may result from buoyancy members that are coupled to the riser sections and are constituted by low-density bodies which are preferably of annular or semi-annular shape to allow an easy handling of those riser sections that have a buoyancy member attached thereto.
During drilling of the hole, it is from time to time required to insert casing into the hole for protecting the hole against collapse of the wall or against inflow of high pressure formation fluid, or for preventing fracturing of the formation. Since the diameter of each additional casing that is inserted through a casing already present in the hole is smaller than the diameter of that latter casing, the diameter of the drilling equipment should also be decreased each time after an additional casing has been set. The riser through which the hole has been initiated often has a very large (up to 30 inches) inner diameter. If desired, this large-diameter riser may be replaced by a riser of smaller diameter in the later stages of the drilling operations when smaller-diameter drilling tools are used. However, since such replacement also requires replacement of additional equipment (such as kill and choke lines, blow-out preventers, telescopic joints, flexible joints), such replacement of the large-diameter riser by a small-diameter one is not considered attractive for economic reasons.
During progress of the drilling operations in a hole, formation pressures of increasing value will be met, which formation pressures call for increasing values of the mud pressure in the hole to maintain the pressure balance in the hole under control. Therefore, the specific gravity of the mud has to be increased from time to time, as a result of which the weight of the mud present in the marine riser is increased. Consequently, the difference between the specific gravities of the mud inside the marine riser and the sea water outside this riser will increase during the drilling operations. As the wall thickness of the riser is dictated by the longitudinal and bending loads to which the riser will be subjected, and the longitudinal loads are dictated by the weight of the riser, by the weight of the mud present in the riser, and by the pressure difference across the wall of the riser, it will be appreciated that the wall thickness of the riser should in particular be sufficiently large to meet the conditions existing in the last stage of the drilling operations, when the specific gravity of the mud has reached its maximum value for the particular drilling operation.
Further, since the mud pressure acting on the inner surface of the riser is being increased during the drilling operations as a result of the above-mentioned increases of the specific gravity of the mud, measures are to be taken to tension the riser sufficiently to prevent buckling thereof. Consequently, the force exerted by the riser tensioner (or heave compensator) will have to be increased each time the specific gravity of the mud is increased.
It has already been proposed in deep-sea operations to provide a tensioned liner string in the marine riser for reducing the flow area in the riser and thereby increasing the flow velocity of the mud to prevent or minimize settling of drill cuttings in the mud stream, and for relieving the riser of high mud pressures.
Object of the invention is a dual riser system for deep sea drilling, wherein a considerable reduction in the capacity of the marine riser tensioner can be obtained.